1. Field of Invention
This invention relates generally to systems for drying microelectronic optical and other parts which in the course of their processing are rendered wet with solvents or other liquids, and more particularly to a system in which the parts to be dried are subjected to a laminar flow of heated gas which promotes uniform evaporative drying of the parts.
2. Status of Prior Art
In critical parts drying systems of the type currently in use in fabricating integrated circuit silicon wafers and other microelectronic devices, in order to achieve optimum cleanliness and a high yield of uncontaminated parts, it is vital that the wet parts be subjected to uniform treatment throughout the work zone of the drying chamber or vessel.
Thus if the parts to be dried are supported within a chamber into which is fed a heated gas to promote evaporative drying, and no means are included to govern or direct the flow of gas, the gas will then flow in a direct line stream extending from the inlet. As a consequence, relatively stagnant pockets of gas will develop in those regions of the chamber which are displaced from the gas stream, and not all of these parts will be adequately dried. Because some of the parts remain contaminated by residual solvents or other liquids, their quality is impaired.
With a view to producing more satisfactory gas flow patterns in a critical parts drying system, it is known to use for this purpose flow diffusers such as perforated screens, baffles, and other expedients to modify or split up the gas stream to create a desirable flow pattern. However, the stringent demands now imposed on process control in microelectronics technology are such that deflectors and other means for modifying gas flow are inadequate, and satisfactory drying results are not realized.
In order to overcome this drawback, my prior U.S. Pat. No. 3,543,776 (Layton) discloses a drying vessel in which the parts to be dried in a work zone are subjected to the flow of a heated inert gas such as nitrogen. The arrangement is such that the gas flow pattern is laminar, thereby effecting a uniform and efficient drying action.
To this end, my prior patent feeds the incoming gas into a pressure chamber disposed below the work zone in the vessel, the chamber being covered by a porous membrane. Under this membrane is a diffuser screen which prevents the gas from directly striking the membrane. As a result, the incoming gas which fills the chamber filters through the membrane into the work zone thereabove in the drying vessel, the membrane producing a pressure differential and hence laminar flow in the work zone.
In practice, the membrane used to establish a laminar flow of nitrogen has a limited ability to serve as an effective particulate filter except for very small gas flow rates. The reason for this limitation is that in all but the smallest pressure chambers, it is necessary that the membrane have a fairly large pore size, otherwise one runs the risk of membrane rupture. To make possible greater flow rates without damaging the membrane, carefully designed membrane supports are provided to this end. But, in general, it was found necessary to augment gas filtration with an auxiliary submicron filter in the gas supply line in all but laboratory scale versions of the drying chamber.
In large commercial scale drying systems of the same type, the pressure chamber below the work zone in the drying vessel was often found to be a source of excessive noise. This noise tended to increase in intensity when the pore size of the membrane was small enough to serve as an effective particulate filter.
The need to use an auxiliary in-line submicron filter did not impose serious limitations on the utility of the evaporative drying chamber until the disadvantages of solvent drying and the environmental problems created thereby led to a growing interest in the use of evaporative drying in areas other than semiconductor and integrated-circuit processing, by means of purified air instead of nitrogen. Thus the precision optics industry and even the eyeglass lens processing industry are at present actively engaged in replacing existing hydrocarbon and fluorocarbon solvent drying systems with other available and cost effective methods.
Though a laminar flow drying chamber of the type disclosed in my prior patent is suitable for those applications entailing the drying of parts other than microelectronic devices, in many cases the cost of nitrogen or even of compressed and purified air has been found in this context to be prohibitive.